landing gear overhauls on n9631r - csobeech.com

6/3/2017 pg 1 of 13 Dynamic Brake Relay Overhaul.docxx

Beech Travel Air Dynamic Brake Relay Overhaul

DYNAMIC BRAKE RELAY ELECTRICAL FUNCTION & CONNECTIONS

The schematic diagram shows brake relay electrical functions and connections arranged roughly as they

correspond to the physical placement of its components. Terminations to circuit breakers, gear limit

switches and the landing gear motor are depicted as well so the connections can be compared to the

wiring diagrams in the Beech manual. The bottom photos show the actual relay from above and below.

DOWN COIL

(RIGHT)

UP COIL

(LEFT)

C1

C22

1

3

RED (DOWN)

TO GEAR

MOTOR

WHITE (UP)

TO GEAR

MOTOR

UP LIMIT

CIRCUIT

DOWN LIMIT

CIRCUIT

LDG GEAR

MOTOR C/B

BLK (GND) TO

GEAR MOTOR

DYNAMIC BRAKE RELAY STATE:A/C ON GROUND

GEAR SELECTED DOWN

GEAR IS DOWN

SHUTTLE SWITCH LATCHED DOWN

ORIG 1/14/2008

REV 1 1/15/2008

REV 2 2/23/2009

REV 3 5/10/2010

C3

SHUTTLE SWITCH

RUN

SWITCHES

BRAKE

SWITCHES

THE RUN SWITCHES ARE OPEN

AND BRAKE SWITCHES CLOSED

WHEN COIL IS INACTIVE

THE SHUTTLE SWITCH

CONNECTS THE

MOTOR BRAKING

WINDING & REMAINS

“LATCHED” AS LAST

POSITIONED BY AN

ACTIVE COIL

GENERAL LAYOUT

Relay is a Cutler-

Hammer type

6046H39A

NOTE:

‘LEFT’ & ‘RIGHT’ COIL

POSITIONS ARE

FACING FWD IN

AIRPLANE

REV 4 6/6/2017


DYNAMIC BRAKE RELAY REMOVAL. MAINTENANCE AND REINSTALLATION

1. The dynamic brake relay is used with the Beechcraft 24 vdc airplanes (Travel Air, Baron and later

Bonanzas) to control the high-amperage landing gear motor running and stopping circuits. This

overcomes problems with other systems like those in Bonanzas that used only the limit microswitches to

control the system directly. The microswitch contacts are unsuited to heavy currents and burn up with

normal use, but the relay contacts are much more robust. Our relay was made by Cutler-Hammer in

1962 as p/n 6046H39A. The photos above show the relay as mounted under the right front passenger

seat next to the flex heater duct (left) and more fully exposed with the duct and gear motor removed

(right). Wiring to the relay connects the related aircraft power circuit breakers, limit switches and the

landing gear motor itself.

2. Begin relay removal by marking all the wiring connections, as shown above left. The motor wiring has

already been removed in this photo. The wires can be disconnected either with the relay mounted or

after removing the two relay mounting screws and lifting the relay from its mounting plate (as shown

above right.) If wires are disconnected in place, be very careful not to drop the small terminal nuts and

washers down into the underfloor areas since access is quite limited.


3. This is how the relay looks once removed and on the workbench. The left photo above shows the relay

positioned as installed in the airplane, looking from the back seat perspective. The right photo shows

the forward side of the relay. Compare these views with the photos at the opening paragraph to better

see other connections, the solenoids and related wiring. Terminals are marked with letters and/or

numbers (C1, C2, etc.) to denote external connections to other system components.

4. The heavy black jumper wire seen in the top photos must be removed from the inboard two top

terminals (above left) to allow removal of the triangular shuttle switch cover and the individual

cylindrical solenoid switch covers. Each circular solenoid switch assembly controls a gear up or gear

down command from the landing gear handle. As shown, “up” is the left coil; “down” is the right. The

photo above right shows the general arrangement of the shuttle switch and the contacts inside the

solenoid switch assembly, including those inside the circular cap. The shuttle switch on the vertical

bakelite piece between them at the back “latches” in the appropriate position to select either the “up” or

“down” motor winding for the braking function when power is removed. When the gear is being raised

the “down” motor winding is selected by the shuttle switch for braking, and vice versa. As shown, the

switch is positioned for “gear down” without power applied to the relay. The shuttle switch arm is

connecting the contacts from upper left to lower right.


5. With both covers removed the internal mechanisms are visible. Each solenoid moves an upper and

lower pair of contacts, insulated by bakelite blocks which are anchored to the solenoid armatures and

ride on threaded guide rods. Unpowered, solenoid springs keep the lower contacts open and the upper

contacts closed. The left solenoid is the “up” control; the right is “down.” When the gear handle is

moved the respective solenoid is energized, closing a lower pair of contacts to operate the gear motor. It

also pulls the associated silvery “teeter totter” lever down, tipping the shuttle switch to bridge the

appropriate pair of motor winding contacts for dynamic braking. Upon reaching the gear travel stop, the

limit switch cuts off the solenoid and the contacts rebound upward to shut off motor power at the lower

contacts and short circuit the inactive motor winding to ground with the upper contacts. This causes a

brief, large reverse current in the motor which stops it almost instantly. Obviously, if contacts are

burned or tarnished their electrical resistance increases. At high currents, even small resistance cause

major voltage loss which hinders motor and braking performance.


6. These photos show more clearly the upper relay contacts and the layout of the lever mechanism that

moves the shuttle switch. The contacts exhibit some tarnish and pitting. Each lever is cushioned with a

small spring so it only pulls down on the shuttle switch cam when its solenoid is activated. The cam

pivot is seen at the hole through the vertical aluminum bracket in the upper right picture.

7. The lower view shows the right solenoid contact package disassembled. All the contacts can be

removed for polishing by carefully removing the small cotter from the metal button atop the lever and

sequentially removing the button, folding back the lever, removing the thin metal washer plus the very

tiny centering washers, spring, contact bars with bakelite separator and the lower fiber disc. All these

parts are shown to the right of the relay. Once opened up like this, the loose contacts can be closely

inspected and polished, and the lower contacts can be removed as well if needed.


8. The lower contacts, shown above, can also be removed to remove pits and polish the surfaces. Simply

remove the nuts from the mounting plate securing the contact (a screw from beneath serves as the

threaded stud. – see addendum 1.) As shown at right, each lower contact segment sits in a machined

groove and is easily removed. NOTE: Pitting on the upper contacts to this extent WILL cause dynamic

braking problems and is not acceptable. All the contact plates and nuts are silver plated and are subject

to tarnishing, as can be seen, but they polish up very well using fine files, mild abrasives or chemical

means.

9. The shuttle switch contacts, shown above and at para. 5, are also subject to tarnish but aren’t as readily

removed. The contact plate is spring loaded so with the relay apart there is good access and it’s easy to

insert a fine point file or piece of Scotchbrite to clean up the contacts. While gently pinching the

contacts together, slide a business card or similar heavy paper between the contacts for final cleaning.


10. With everything apart, it’s a good time to brush or blow away dust and debris accumulated in the many

nooks and crannies of the relay. Wipe the exterior covers clean and scrub the external connection

terminals to remove tarnish and dirt.

11. Reassembly was essentially the reverse process, with careful reference to photographs to arrange

washers, springs and other small items in their original configuration. The most tedious part of the

process is replacing the tiny washers and spring under each solenoid lever, and then inserting the cotter

pins into the small metal buttons. The pins fit rather tightly so if the holes aren’t well aligned or the

cotters aren’t straight the cotters won’t go in. Double-check that the shuttle switch is placed in the gear

down position, with the contact pivot running from upper left contact to the lower right contact (roughly

10 o’clock to 4 o’clock) as seen at the top of the lower photo on the previous page.


The wiring can be reconnected to the relay either after the relay is mounted back in its tray in the airplane,

or beforehand with the relay on the cabin floor just aft of its under-seat location. Clean all the wire

terminals. Before mounting the relay in the tray it’s a good idea to clean the area of the debris that always

accumulates, and be sure to clean the ground connections. Access in the mounted position is poor and if a

nut or washer is dropped into the underfloor area it may be hard to find without removing everything again.

A small cloth laid underneath will help avoid losing dropped items. The photos on the previous page show

the locations and engraved markings of the terminal connections. The next annotated photo shows which

functions are connected to each terminal.


12. These photos depict the relay mounting tray and the relay installed on it. Access is limited by the gear

actuation rods and fuel lines, partly visible. The circular heater duct tee is visible in the left photo.

Secure the relay in the tray with its two ground screws; note that 2 wires go to the left screw. Assuming

the gear motor is already installed reinstall the flex heater duct to the metal tee. Install the aft upholstery

panel and connect the flex duct to its heat outlet with the remaining worm clamp.

13. To complete functional checks, refer to the shop manual procedures about landing gear functional

checks. Finally, close up the spar access panels, replace the aft cabin carpet and reinstall the front seats.

With suitable inspection and logbook entries from the A&P, the job is done.

ALTERNATE RELAY PART NUMBERS

According to aircraft salvage house data and FAA service reports, the following relays are more or less

interchangeable to use for the same function:

Cutler-Hammer (Eaton) 6046H39A (as installed in Travel Air and listed in parts book),

Cutler-Hammer (Eaton) 6046H39B, or Cutler-Hammer (Eaton) 6046H58

Cutler-Hammer (Eaton) SM50D7 (used on

F33A series with numerous SDR write-ups.)

Very similar design using a plastic housing

for the contacts, as shown at right.


DYNAMIC BRAKE RELAY ADDENDUM 1 - June 2017

In May 2017 the dynamic brake relay was again overhauled. This section discusses added

information not provided in the earlier pages.

1. The solenoid contacts can be effectively cleaned and polished using a small brass rotary

wire end brush in an electric drill. This brings the contact surface to higher degree of polish

than Scotchbrite or fine sandpaper. Surface roughness is easily removed with light filing first.

2. The shuttle switch assembly can be removed by taking out two screws under the bakelite

plate via the access holes through the relay chassis, as shown. The visible screw threads into

the shuttle switch base itself. The obscured screw threads into a small attach plate which

secures the shuttle and positively locates the bakelite covers over the solenoid contacts.

The shuttle switch footprint is seen below after removal with its 2 mounting screw holes.


These photos show the shuttle switch assembly in more detail. The attachment plate is tapped

for one of the attaching screws (shown) and provides secure orientation for the cylindrical

solenoid contact covers seen in previous pages. The fixed contacts were badly pitted (first

photo) but light filing followed with fine sandpaper and then rotary brushing with the brass

drill brush brought them to a high polish. The moving contacts on the shuttle arm were

polished with fine sandpaper between the contact surfaces by lightly pressing the arm against

the fixed contacts while moving the sandpaper.


3. The lower fixed contacts shown earlier are easily removed for cleaning as shown below.

The first photo shows the pitted contacts before removal. First take off the contact nut (top

right photo), then loosen the attaching screw from beneath the base plate. The screw threads

into the contact itself, shown lower left. The final photo shows the lower fixed contact after

cleaning next to its mounting screw, and the mating movable lower contact plate to the left.

4. A good ground connection is critical to proper braking function. The photo shows the

relay tray ground screw location, which should be cleaned of paint for the best conductivity.


DYNAMIC BRAKE RELAY ADDENDUM 2

General Notes on 24 vdc Landing Gear Motor & Gearbox Operation

1. The landing gear motor turns about 7500 rpm for 4-5 seconds to extend or retract the

gear. This equates to about 7500*4/60 = 500-625 revolutions of the motor.

2. The landing gear gearbox rotates roughly ½ turn for about 500 turns of the motor

(rounded off) or 50 turns of the hand crank. This is a 1000:1 reduction for the motor

and a 100:1 reduction for the hand crank.

3. When the landing gear is operated the travel limit settings are expressed in terms of

1/8 to ¼ turn of the hand crank away from mechanical stops. Since this is related to

50 crank turns to produce about 180 degrees or ½ turn of sector gear rotation, the stop

tolerances equate to about 1/8*1/100 = 1/800 to 1/400 of a full rotation of the sector

gear, or roughly 360/800 = about ½ to 1 degree of sector rotation.

4. Assuming 3/4 degree of sector rotation margin for 3/16 turn of hand crank (midway

between 1/8 and ¼ turn) before the mechanical stop is reached, and a sector gear

diameter of 12 inches, the amount of slack stopping distance around the circumference

of the sector gear is roughly 3/16*12*3.1416*1/100 = .071 inch.

5. Figuring the opposite way back to the motor, this tolerance requires the motor to come

to a complete stop from 7500 rpm leaving roughly 3/16*1000/100 = 2 turns of the

motor before reaching the mechanical stops. This implies a motor rotational accuracy

error of 2/500, about 0.4%. If the limit microswitches electrically actuate 1/8 inch

before the sector gear actually stops, at a switch radius point of about 4 inches, this

means the motor coasts about 5 turns during the braking process (1000/(8*8*3.1416)

= 5), taking about 1/12 second (assume 7500 rpm linear deceleration to zero, average

= 3750 rpm; 5*60/3750 = .08 second.)

6. Paragraph 5 makes it clear why low resistance in the dynamic brake circuit is so

critical to proper gear limit adjustments. To stop the motor so quickly the braking

surge current is very high, say 10 times the normal operating level. If the circuit

resistance is, say, only 0.05 ohms, either at the motor connections, motor brushes or

the brake relay contacts, this would still pass most of the normal running voltage at 20

amps (24 – 20*0.05 = 23 vdc). So the gear transit times would be reasonable. But for

braking, the 0.05 ohms would dissipate much of the induced reverse voltage at 200

amps (200 * 0.05 = 10 vdc lost) so stopping quickly and consistently becomes

problematic. NOTE: This was confirmed in actual experience, when only one of

four upper relay contacts was found pitted, that being enough to ruin dynamic

braking at the “up” gear limit even after a shop overhaul of the gear motor.

Dressing the contacts smooth completely solved the problem.

landing gear overhauls on n9631r - csobeech.com

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